This invention relates to apparatus for supplying power to electrical devices having conductors cooled to low temperatures in general and more particularly to an improved type of apparatus of this nature in which the normal conductors therein need not be overdesinged and coolant losses are reduced.
It is often necessary to supply power to electrical devices having conductors which are cooled to low temperatures from a power supply which is at a higher temperature such as room temperature. This is the case particularly with respect to devices utilizing superconductors such as superconducting cables, coils, or machines which contain superconductors which must be cooled to a temperature below the transition temperature of the superconductive material. Because a superconductor losses its superconductivity at a temperature far below room temperature, metal having a normal electrical conductivity can be used to bridge the temperature difference. Typical of such are aluminum and copper. Such a normal conductor will be connected to the superconductor at a point which is kept below the transition temperature of the superconductor. The normal conductor in turn can be cooled gradually or in steps from the room temperature to the low temperature at the junction point.
Stepwise cooling of this nature in an apparatus supplying power to cooled conductors is described in U.S. Pat. 3,764,726. In the disclosed apparatus several heat exchangers which are at fixed temperature levels are employed.
In addition to apparatus of this nature in which cooling takes place in cascade fashion, apparatus supplying power to cooled conductors can be of type continuous cooled by an exhaust gas. Such devices are frequently used because of better heat exchange conditions and their simpler design. In such an apparatus the superconductor end which must be kept below the transition temperature can be disclosed in a bath of cryogenic medium such as a liquid helium bath. In such a case the normal conductor can consist, at the junction point, of individual wires, lamanations or screens. An embodiment of this nature for supplying large currents is described in "The Review of Scientific Instrument, " Vol. 38, No. 12, Dec. 1967, pages 1776 to 1779. The liquid helium in the bath is evaporated partially by the thermal losses of the current supply components. The helium rises along the conductor laminations, wires or conductor screen and dissipates joule heat along with the heat flowing in from the outside. As a result the helium gas heats up to approximately room temperature. To improve heat dissipation the helium bath can also be equipped with an additional heat source to establish higher evaporation rates. At the upper point of contact between the normal conductor in the current supply apparatus and external current supply line the helium gas is generally captured and then conducted to a refrigeration system where it is reliquified. Since the heat content of the gaseous coolant is well utilized in such current supply apparatus in which cooling by exhaust gas takes place, the cost for coolant requirements is relatively low.
The cross-sectional area of the normal conductor in such a current supply which is cooled by exhaust gas can be optimized for a certain nominal current. At this nominal current the liquid helium losses of the current supply are at a minimum and a continuous temperature gradient becomes established along the conductor from the hot end at room temperature to the cold end in the helium bath. it has been found in practice, however, that such ideal operating conditions cannot always be maintained. With only a light excess current the continuous temperature gradient along the conductor will be lost. Instead a steep temperature increase occurs at first, and only after a certain conductor length, which depends on the excess current, does the temperature gradient again decrease continuously toward the cold end of the conductor. As a result in the conductor section of increasing temperature the danger that the electrical insulation surrounding it can become damaged or that the conductor can melt exists. Because of this it is common practice to make the cross-sectional area of the conductor larger than would be required for a given nominal current as a safety measure. In addition to the overdesign of conductor with regard to its cross-section the undesired temperature increases in the conductor can be avoided by providing means to insure that additional quantities of the cooling gas are generated using a heating element in the helium bath. This, however, results in the temperature at the hot end of the conductor dropping so that the dielectric high voltage strength of the insulator is endangered by condensed water forming at this point. As a result an additional heating system may be required to avoid the formation of condensate at this point. The consequences of taking measures of this nature is that the quantity of liquid helium required goes beyond the optimum for such a current supply.
Furthermore an enlargement of the cross section results in an increased introduction of heat into the coolant bath so that the coolant losses will be greater even under no load operating conditions, i.e. even when no current is flowing through the conductor, the increased size thereof conducts more heat from the outside room temperature to the coolant bath.
In view of these difficulties it is the object of the present invention to provide an improved current supply apparatus of this nature in which the above noted problems are reduced or even eliminated.